Apparatus and method for applying feedback control to a magnetic lens

ABSTRACT

An apparatus configured to control a magnetic field strength of a magnetic lens is provided. The apparatus may include a magnetic sensor configured to generate an output signal responsive to a first magnetic field strength of the magnetic lens. The apparatus may also include a control circuit coupled to the magnetic sensor and the magnetic sensor. The control circuit may be configured to receive the output signal from the magnetic lens and to receive an input signal responsive to a predetermined magnetic field strength. The control circuit may be further configured to generate a control signal responsive to the output signal and the input signal. Additionally, the control circuit may be configured to apply a current to the magnetic lens such that a second magnetic field strength may be generated within the magnetic lens closer to the predetermined magnetic field strength than the first magnetic strength.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention generally relates to a magnetic lens which may beconfigured to apply a magnetic field to a charged particle beam, andmore particularly, to a sectored magnetic lens and a control apparatusfor a magnetic lens which may be incorporated into a scanning electronmicroscope system.

[0003] 2. Description of the Related Art

[0004] As the dimensions of semiconductor devices continue to shrinkwith advances in semiconductor materials and processes, the ability toexamine microscopic features and to detect microscopic defects hasbecome increasingly important in the successful fabrication of advancedsemiconductor devices. Significant research continues to focus onincreasing the resolution limit of metrology tools that are used toexamine microscopic features and defects. Optical microscopes generallyhave an inherent resolution limit of approximately 200 nm and havelimited usefulness in current manufacturing processes. Microscopes thatutilize electron beams to examine devices, however, may be used toinvestigate feature sizes as small as, e.g., a few nanometers.Therefore, tools that utilize electron beams to inspect semiconductordevices are increasingly becoming integral to semiconductor fabricationprocesses. For example, in recent years, scanning electron microscopyhas become increasingly popular for the inspection of semiconductordevices. Scanning electron microscopy generally involves scanning anelectron beam over a specimen and creating an image of the specimen bydetecting electrons that are reflected, scattered, and/or transmitted bythe specimen.

[0005] The electron optical system of a scanning electron microscopegenerally includes an electron source, a device or a plurality ofdevices configured to focus an electron beam generated by an electronsource, a detector or a plurality of detectors configured to detectelectrons reflected, scattered, or transmitted by the specimen, and acontrol system. A thermal field emission source may typically be used asan electron source, and the energy of the electron source may becontrolled by an emission control electrode and an anode. The electronbeam may pass through a magnetic condenser lens configured to collimatethe electron beam. An initial deflection system may also be located nearthe electron source. An initial deflection system may be configured tocorrect alignment, stigmation and blanking of the beam. Prior to passingthrough a magnetic objective lens, the beam may also be passed through abeam limiting aperture and one or more electrostatic pre-lensdeflectors. The magnetic objective lens may further focus the electronbeam to a spot size of, for example, approximately five nanometers. Asused herein, the term “spot size” is generally defined as a lateraldimension of an electron beam incident upon a specimen. A magneticobjective lens may typically include a lower pole piece, an intermediateelectrode, and an upper pole piece.

[0006] An electron beam exiting a magnetic objective lens may be scannedacross a specimen. Typically, the electron beam may be scanned in afirst direction while the stage supporting the specimen may be moved ina direction perpendicular to the first direction. A plurality ofdetection systems may be used to detect secondary electrons,back-scattered electrons, and transmitted electrons that may be producedwhen the electrons contact the specimen. Examples of scanning electronmicroscope systems are illustrated, for example, in U.S. Pat. No.4,928,010 to Saito et al., U.S. Pat. No. 5,241,176 to Yonezawa, U.S.Pat. No.5,502,306 to Meisburger et al., U.S. Pat. No. 5,578,821 toMeisburger et al., U.S. Pat. No. 5,665,968 to Meisburger et al., U.S.Pat. No. 5,717,204 to Meisburger et al., U.S. Pat. No. 5,869,833 toRichardson et al., U.S. Pat. No. 5,872,358 to Todokora et al., and U.S.Pat. No. 5,973,323 to Adler et al., and are incorporated by reference asif fully set forth herein.

[0007] The performance of a scanning electron microscope may varydepending on, for example, the capability to focus an electron beam on asmall target area. High voltage electrons may penetrate deep into asemiconductor substrate or a portion of a semiconductor formed upon asemiconductor substrate thereby damaging the substrate or the device andrendering it unsuitable as a working device such as an integratedcircuit. Therefore, low voltage electron beams may typically used toanalyze delicate semiconductor specimens that otherwise might be damagedby high voltage electron sources. The primary factor that reducesresolution in the low acceleration voltage region is blur of theelectron beam due to chromatic aberration. Dispersion in the energy ofthe electron beam emitted from the electron source typically causeschromatic aberration. As such, significant effort has been focused onimproving the performance of a scanning electron microscope by enhancingthe ability of the magnetic objective lens to reduce chromaticaberrations in an electron beam source especially in low voltageparticle beams.

[0008] Traditionally, magnetic lenses may be axially symmetric and mayproduce axially symmetric magnetic potentials and magnetic fields. Anexample of such a magnetic lens is illustrated, for example, in U.S.Pat. No. 6,002,135 to Veneklasen et al. and is incorporated by referenceas if fully set forth herein. A magnetic lens may include an inner polepiece that may have a cylindrical upper portion and a conical lowerportion that may be substantially enclosed by an outer pole piece. Theouter pole piece may also have a cylindrical upper portion and a conicallower portion corresponding to the inner pole pieces. A solenoidalexcitation coil may be disposed between the inner pole piece and theouter pole piece. When a current is applied to the excitation coil, anaxial focusing field may be generated within the lens by magnetic fluxfrom the inner and outer pole pieces. The axial focusing field may beused to focus an electron beam. Shielding rings may be arranged betweenthe upper and lower portions of the inner pole piece to reduce the airgap between the pole portions. The shielding rings may also provide areturn path for deflection flux that may otherwise radiate through thegap and induce eddy currents in outer pole pieces and excitation coil.Deflection coils may also be included within the lens along the beampath.

[0009] Variable axis lenses have also been developed to focus electronbeams. Variable axis lenses incorporate supplementary lenses orsupplementary deflectors in the magnetic lens to provide some correctionof electron beam paths that may be laterally displaced from an opticalaxis of the lens. The supplementary lenses and deflectors may beenergized based on the lateral displacement of the beam path. Althoughelectron beams may be deflected by this lens, astigmation may still be aproblem. Therefore, a separate astigmation compensator may also beincluded in such a lens. Alternatively, an astigmatism-correctiondeflector system may be arranged within a variable axis lens adjacentthe internal surface of the supplementary deflectors. Such deflectorsmay be constructed of an octapole three-stage coil in which eachoctapole includes two tetrapole sets. A deflection field coil may beadded to one of the tetrapole coil sets of the octapole. An example of avariable axis lens is illustrated in U.S. Pat. No. 5,952,667 to Shimizuand is incorporated by reference as if fully set forth herein. Theincorporation of a separate astigmator octapole forces the beam to passthrough the center of this octapole. The overall alignment of the lenssystem, however, may be non-colinear due to the incorporation of such aseparate feature. Therefore, complexity of the overall alignment of thesystem increases when the charged particle beam is forced to passthrough successive non-colinear points.

[0010] There are, however, several disadvantages to the lens systemsdescribed above. For example, axially symmetric lenses may typicallysuffer from hysteresis, large inductance of the excitation coil, andthermal stability problems. Hysteresis may cause a relationship betweenthe excitation coil current and the deflected beam position to dependupon past deflection history. Therefore, accurate focus of an electronbeam using the magnetic lens may be extremely difficult to maintain andcontrol. Additionally, large inductance of the excitation coil may causethermal stability problems due to heat generated by the lens or theexcitation coils. Therefore, the center of the lens may shift due tothermal expansion of the materials used to construct the lens.

[0011] Immersion lenses are also limited in their application to avariety of specimens. Immersion lenses are generally designed to limitaberrations of an electron beam by reducing the distance between thespecimen and the maximum magnetic field. The distance between thespecimen and the maximum magnetic field may be reduced by placing thespecimen near or within the lens. Examples of immersion lenses areillustrated in U.S. Pat. No. 5,089,428 to Da Lin et al. and isincorporated by reference as if fully set forth herein. Due to spacelimitations, immersion lenses may not be able to accommodate a largespecimen such as a semiconductor substrate. For example, 200 mm wafers,or semiconductor substrates, are already being used in the developmentand production of semiconductor devices. Efforts are also underway tofurther increase the size of semiconductor substrates to 300 mm.Modifying these lenses in order to accommodate such large semiconductorsubstrates may also adversely affect the performance of immersionlenses. Alternatively, reducing the size of the semiconductor substrateby cross-sectioning the wafer is not usually an option due to the costassociated with destroying a product wafer.

[0012] An asymmetric immersion lens may be configured to reduce thedistance between a specimen and the strongest magnetic field of thelens. An asymmetrical lens, however, may be configured to produce amagnetic field that rises sharply just in front of a conical pole piecenear the bore of the lens or the position at which the electron beamexits the magnetic lens. The magnetic field falls slowly toward a secondpole piece or a magnetic housing. The specimen and the conical polepiece are disposed within the magnetic housing such that the specimenmay be placed near the conical pole piece. Asymmetric immersion lensesmay be more flexible to accommodate large specimen such as semiconductorsubstrates, but these lenses may have a reduced capability to detectsecondary electrons. For example, because secondary electrons may beemitted at a point beyond the magnetic field peak, low energy secondaryelectrons may not be able to surmount the magnetic field maximum. Inorder to overcome low detection of secondary electrons, a conductinggrid system may be included in the lens. The conducting grid system mayinclude an auxiliary grid to accelerate the secondary electrons awayfrom the inner surface of the lower pole piece, an extraction grid toreduce the axial velocity of the secondary electrons, and a restraingrid to turn back any uncollected secondary electrons. Therefore, inorder to overcome the low detection of secondary electrons, a conductinggrid system may be included in the lens.

[0013] In addition to the above disadvantages, the performance ofmagnetic lenses may also be limited due to changes in the magnetic fieldstrength due to low frequency noise, drift in the performance of currentdrive electronics, drift due to eddy currents or superimposed fieldsfrom other sources, and drift in the magnetic field strength over timefrom other causes. Although a magnetic lens design may minimize theseeffects on the performance of the lens, it may not be possible tosubstantially eliminate magnetic field drift of the lens. For example,eddy currents due to magnetic flux leakage from a lens through a gap inthe magnetic lens may adversely affect the performance of the magneticlens. Because a magnetic lens must be designed with a bore to allow theelectron beam to travel through the lens, however, it is impossible toseal the magnetic lens off completely. As a result, some of the magneticfield will inherently “leak” out of a magnetic lens. Therefore, theeffects of eddy currents on the performance of a magnetic lens may notbe completely eliminated due to usage requirements. Drift in themagnetic field may cause the electron beam to drift out of focus.Therefore, the overall resolution of a scanning electron microscope mayalso be reduced by the presence of the above sources of magnetic fielddrift. The functioning of the scanning electron microscope may, however,be dramatically improved by an accurate control system for the magneticobjective lens.

[0014] A control mechanism for a magnetic lens may generally include adevice for sensing the current density of the electron beam at aposition spaced from an axis along which an electron beam travels. Forexample, an alignment yoke disposed along the axis of the electron beammay receive a signal from the current sensing arrangement. Therefore,the alignment yoke may be mechanically shifted incrementally andorthogonally until a maximum current may be produced at the referencelocation. The beam may be centered due to the altered position of thealignment yoke thereby reducing aberrations in the projections lens. Anexample of such a focusing system is illustrated in U.S. Pat. No.4,423,305 to Pfeifer and is incorporated by reference as if fully setforth herein. Mechanical focus methods, however, may be slow due to thetime required for moving the devices. In addition, microscopicvibrations due to mechanical motion of the alignment yoke may need tosettle before the magnetic lens may perform adequately. Therefore,mechanical focus methods may require additional time such thatmicroscopic vibrations will not affect the performance of the magneticlens.

[0015] An alternative control mechanism for a magnetic lens involvesfocusing and controlling an electron beam by determining the focallength at which a sample will be brought into focus. Focal length may bea function of the electron beam energy and the magnetic field strength.In this manner, one available control mechanism involves using anelectron trajectory tracing program to measure the converging point, orfocal length, for an electron beam by using measurements of the electronbeam energy and the magnetic field strength. The magnetic field strengthmay be estimated by measuring a current in the lens coil. An adjustmentto the current in the lens coil may be made to correct the convergingpoint of the electron beam. Instantaneous magnetic field strength,however, may be determined by the present current and the history of allother currents in the coil which is commonly referred to as hysteresis.Hysteresis in the magnetic field strength may also be induced fromfrequent changes in the voltage supplied to a magnetic lens. Forexample, the lens current in a scanning electron microscope may often beautomatically adjusted to a nominal beam voltage. As new specimens arebeing observed, the user may usually alter the beam voltage to bring thespecimen into focus. Therefore, frequently adjusting the beam voltagemay increase the complexity of a current history of the lens. As thecomplexity of the current history increases, hysteresis will also becomemore problematic. Therefore, estimating the magnetic field strengthusing measurements of the current in the lens coil may induce error inthis method. In addition, the coil and the core materials may react in anon-ideal way to the frequent changes in the current being supplied tothe coil. Therefore, additional electrical and magnetic characterizationof the coil and the core materials may be necessary. A thoroughdegaussing procedure may reduce the effects of hysteresis, however,magnetic hysteresis typically remains a problem in most magnetic lenses.

[0016] Additional methods to control the electron beam focus haveattempted to reduce the effects of hysteresis of the magnetic lens bykeeping the lens current constant after an initial manual focus andcalibration. An example of such a magnetic lens control method isillustrated in U.S. Pat. No. 4,999,496 to Shaw et al. and isincorporated by reference as if fully set forth herein. The controlmethod involves varying the electron beam energy to alter the focus ofthe magnetic lens, as working distance changes such as when differentareas of a specimen are viewed. In order to offset the effects of a newbeam energy, the current in the scanning coils may be altered tomaintain accurate magnification. Although such a method for focusing theelectron beam may reduce deleterious effects of hysteresis in thecurrent of the magnetic objective lens, other factors that may lead todefocusing may not be addressed in this design. For example, asmentioned above, other factors that may hinder the performance of amagnetic lens may include thermal changes in the material properties,drift in the current drive electronics, drift in the magnetic field dueto eddy currents, and drift due to superimposed fields from othersources.

[0017] Furthermore, in many scanning electron microscope systems, coarseand fine focusing of the magnetic lens may typically be performedmanually by an operator. The operator may alter the focus of themagnetic lens by controlling the electric current of the magnetic lens.In order to obtain the desired magnetic field strength, the operator mayalter the current while observing the effects of the magnetic field onan image of a specimen until the optimal performance is achieved. Theimage may be observed using a display system such as a color orgrayscale monitor. The resulting magnetic field, however, may follow anonlinear relationship with the current due to hysteresis in themagnetic lens and the behavior of the coil and the core materials inresponse to the change in current. This iterative method also depends onoperator judgement and is consequently subject to error. Moreimportantly, the additional sources of magnetic field drift describedabove may also cause this magnetic field to be irreproducible.

[0018] Accordingly, it would be advantageous to improve the performanceof magnetic lens and a magnetic lens control apparatus that may be usedto focus an electron beam by reducing the effects of, for example,hysteresis and thermal instability in the magnetic field strength of amagnetic lens.

SUMMARY OF THE INVENTION

[0019] In an embodiment, a magnetic lens such as a magnetic circuitconfigured to apply a magnetic field to a charged particle beam and anapparatus configured to control a magnetic field strength of a magneticlens are provided. The charged particle beam may be a beam of ions or abeam of electrons. The charged particle beam may travel through themagnetic lens from a first end of the magnetic lens to a second end ofthe magnetic lens. The magnetic lens and the apparatus may be configuredto be incorporated into a scanning electron microscope.

[0020] In an embodiment, the magnetic lens may have an outer pole pieceand an inner pole piece. The outer pole piece may be coupled to theinner pole piece. The outer pole piece may have at least two sectors andat least two slots. In addition, the outer pole piece may have eightsectors and eight slots. Each sector may be disposed between lateralboundaries of two slots in the outer pole piece such that the magneticpotential of each sector may be substantially independent of themagnetic potential of each other sector on the outer pole piece.Furthermore, the inner pole piece may also have at least two sectors andat least two slots. In addition, the inner pole piece of the magneticlens may have eight sectors and eight slots.

[0021] In an embodiment, the magnetic lens may include a primary coilwinding. The primary coil winding may be interposed between the outerpole piece and the inner pole piece of the magnetic lens. The primarycoil winding may be configured to drive a magnetic potential of theouter pole piece relative to the inner pole piece when a current isapplied to the primary coil winding. The magnetic lens may also includeat least two sector coil windings. Each sector coil winding may becoupled to one sector of the outer pole piece. In addition, if the innerpole piece also has sectors, then a sector coil winding may be coupledto each sector of the inner pole piece. In this manner, the magneticlens may have an equal number of sectors and sector coil windings. Eachsector coil winding may be configured to drive a magnetic potential ofthe sector coupled to each sector coil winding, respectively, when acurrent is applied to each sector coil winding. Therefore, the magneticpotential of each sector may include the magnetic potential generated bythe current applied to primary coil winding which may affect each sectorsubstantially equally. The magnetic potential of each sector may alsoinclude the magnetic potential generated by the current applied to eachrespective sector coil winding. As such, each sector may have a magneticpotential that may be substantially independent of the magneticpotentials of the other sectors of the magnetic lens. The magnetic fieldthat may be applied to the charged particle beam, therefore, may includethe magnetic potential of the outer pole piece relative to the innerpole piece and the magnetic potential of one or more sectors of themagnetic lens.

[0022] In an embodiment, a method for applying a magnetic field to acharged particle beam may include directing the charged particle beamthrough a magnetic lens. Directing the charged particle beam may includepositioning a charged particle beam source configured to generate acharged particle beam in substantial alignment with a first end of themagnetic lens. The charged particle beam may travel from a first end ofthe magnetic lens to a second end of the magnetic lens. The magneticlens may be configured as described in an above embodiment. The methodmay also include applying a first current to a primary coil winding ofthe magnetic lens to drive a magnetic potential of an outer pole pieceof the magnetic lens relative to an inner pole piece of the magneticlens. In addition, the method may include applying a second current toat least one sector coil winding of the magnetic lens to drive amagnetic potential of at least one sector of the magnetic lens.Therefore, the magnetic field that may be applied to the chargedparticle beam may include the magnetic potential of the outer pole piecerelative to the inner pole piece and the magnetic potential of at leastone sector of the magnetic lens.

[0023] In an embodiment, a method for focusing a charged particle beamon a specimen is provided. The specimen may be a semiconductor devicethat may be fabricated using a semiconductor fabrication process.Alternatively, the specimen may be a portion of a semiconductor deviceor another specimen such as a biological sample. The method may includepositioning at least a portion of the specimen in a path of the chargedparticle beam. The charged particle beam may be directed through amagnetic lens from a first end of the magnetic lens to a second end ofthe magnetic lens. The magnetic lens may be configured as described inan above embodiment. Focusing a charged particle beam on a specimen mayalso include applying a first current to a primary coil winding of themagnetic lens to generate a magnetic potential of an outer pole piece ofthe magnetic lens relative to an inner pole piece of the magnetic lens.In addition, focusing a charged particle beam on a specimen may includeapplying a second current to one sector coil winding of the magneticlens to generate a magnetic potential of one or more sectors of theouter pole piece of the magnetic lens. In this manner, the magneticfield that may be applied to the charged particle beam may include themagnetic potential of the outer pole piece relative to the inner polepiece and the magnetic potential of one or more sectors of the magneticlens. The method may also include altering the magnetic potential of theouter pole piece to apply a coarse focus adjustment to the chargedparticle beam. Furthermore, the method may include altering the magneticpotential of one or more sectors to apply a fine focus adjustment to thecharged particle beam.

[0024] In an embodiment, a system that may be used to inspect asemiconductor device is provided. The semiconductor device may befabricated on a semiconductor substrate using a semiconductormanufacturing process. The system may be a scanning electron microscopethat may use an electron beam to inspect the semiconductor device. Thesystem, however, may be any system that may use a charged particle beamsuch as a beam of electrons or a beam of ions to inspect a semiconductordevice. The system may include a charged particle beam source configuredto generate a charged particle beam. The system may also include atleast one magnetic lens that may be configured as described herein toapply a magnetic field to the charged particle beam. The magnetic lensmay be positioned along a path of the charged particle beam generated bythe charged particle beam source. As such, the charged particle beam maypass through the magnetic lens from a first end of the magnetic lens toa second end of the magnetic lens. The system may further include astage configured to support at least a portion of the semiconductordevice. The stage may be positioned along the path of the chargedparticle beam such that the charged particle beam may interact with thesemiconductor device. The charged particle beam may interact with thesemiconductor device subsequent to having a magnetic field applied tothe charged particle beam by the magnetic lens.

[0025] In an additional embodiment, a method for inspecting a specimensuch as a semiconductor device is provided. Inspecting the specimen mayinclude positioning at least a portion of the specimen on a stage. Thestage may be configured to support the specimen and may be positionedalong a path of the charged particle beam. The method to may includegenerating a charged particle beam. The generated charged particle beammay be directed through at least one pole piece of a magnetic lens suchthat the charged particle beam may travel from a first end of themagnetic lens to a second end of the magnetic lens. The magnetic lensmay be configured as described herein and may be incorporated into ascanning electron microscope. The method may further include applying afirst current to a primary coil winding to generate a magnetic potentialof an outer pole piece of the magnetic lens relative to an inner polepiece of the magnetic lens. In addition, the method may include applyinga second current to a sector coil winding of the magnetic lens togenerate a magnetic potential of at least one sector of the outer polepiece. Therefore, the magnetic field applied to the charged particlebeam may include the magnetic potential of the outer pole piece and themagnetic potential of one or more sectors of the outer pole piece.

[0026] An additional embodiment relates to a computer-implemented methodfor controlling a magnetic field that may be applied to a chargedparticle beam. The computer-implemented method may be implemented bycontroller software that may be executable on a controller computer. Thecontroller computer may be coupled to a magnetic lens. The method forcontrolling a magnetic field may also be implemented by programinstructions that may be incorporated into a carrier medium. The methodmay include measuring a magnetic field generated within a magnetic lensthat may be configured as described herein. The charged particle beammay be directed through the magnetic lens such that the magnetic fieldof the magnetic lens may be applied to the charged particle beam.

[0027] The method may also include determining a primary current inresponse to the measured magnetic field. The primary current may beapplied to a primary coil winding that may be coupled to a pole piece ofthe magnetic lens. In this manner, a magnetic potential of the polepiece may be generated. In addition, the method may include determininga secondary current in response to the measured magnetic field. Thesecondary current may be applied to at least one sector coil windingcoupled to one sector of the pole piece. As such, a magnetic potentialof each sector of the pole piece may be generated. Therefore, themagnetic field applied to the charged particle beam may include themagnetic field strength of the pole piece of the magnetic lens and themagnetic field strength of one or more sectors of the magnetic lens. Themethod may further include controlling the applied primary current andthe applied secondary current. Controlling the applied primary currentand the applied secondary current may be performed while the magneticlens is being used.

[0028] In an embodiment, an apparatus configured to control a magneticfield strength of a magnetic lens is provided. The apparatus may includea magnetic sensor disposed within a magnetic field generated by themagnetic lens. The magnetic sensor may be configured to generate anoutput signal that may be responsive to a first magnetic field strengthof the magnetic lens. The apparatus may also include a control circuitthat may be coupled to the magnetic sensor and the magnetic lens. Thecontrol circuit may be configured to receive the output signal from themagnetic sensor and an input signal that may be responsive to apredetermined magnetic field strength. A manually-controlled operatingsystem or a computer-controlled operating system may be coupled to theapparatus and may be configured to generated the input signal. Inaddition, the control circuit may be configured to generate a controlsignal that may be responsive to the output signal and the input signal.Furthermore, the control circuit may be configured to apply a current tothe magnetic lens. The current may be responsive to the control signal.Therefore, the applied current may be effective to generate a secondmagnetic field strength within the magnetic lens. The second magneticfield strength may be closer to the predetermined magnetic fieldstrength than the first magnetic field strength. In addition, the secondmagnetic field strength may be approximately equal to the predeterminedmagnetic field strength.

[0029] In an embodiment, a magnetic lens such as a magnetic lens asdescribed herein may be configured to apply a magnetic field to acharged particle beam. The magnetic lens may be positioned along a pathof the charged particle beam such that the charged particle beam maypass through the magnetic lens. The charged particle beam may be anelectron beam or an ion beam. As such, the apparatus as described hereinand the magnetic lens may be coupled to a system such as a scanningelectron microscope. The system may also include a charged particle beamsource that may be used to produce the charged particle beam. Inaddition, the system may further include a stage configured to supportat least a portion of a specimen. The stage may also be positioned alonga path of the charged particle beam such that the charged particle beammay interact with the specimen. The apparatus and the magnetic lens may,therefore, be used to inspect a specimen such as at least a portion of asemiconductor device, which may be fabricated by using a semiconductorfabrication process.

[0030] In an additional embodiment, a method for controlling a magneticfield strength of a magnetic lens is provided. The method may beperformed substantially continuously or substantially intermittently.The method may include generating an output signal in response to afirst magnetic field strength generated by the magnetic lens. The methodmay also include receiving an input signal. The input signal may begenerated in response to a predetermined magnetic field strength.Furthermore, the method may include generating a control signal inresponse to the output signal and the input signal. Additionally, themethod may also include applying a current to the magnetic lens. Theapplied current may be applied in response to the control signal.

[0031] In a further embodiment, a method for inspecting a specimen isprovided. The specimen may include at least a portion of a semiconductordevice. The specimen may be positioned along a path of the chargedparticle beam by a stage configured to support the specimen. The methodmay include generating a magnetic field by a magnetic lens and applyingthe magnetic field to a charged particle beam. Applying the magneticfield to the charged particle beam may include directing the chargedparticle beam through the magnetic lens. The charged particle beam maybe an electron beam or an ion beam. The charged particle beam may begenerated by using a charged particle beam source. Therefore, themagnetic lens may be coupled to a system such as a scanning electronmicroscope. In addition, the method may include controlling a magneticfield strength of the magnetic lens as described in the aboveembodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Other objects and advantages of the invention will becomeapparent upon reading the following detailed description and uponreference to the accompanying drawings in which:

[0033]FIG. 1 depicts a perspective view of an embodiment of a magneticlens including a pole piece having eight sectors and eight slots;

[0034]FIG. 2 depicts a side view of an embodiment of a pole piece thathas eight sectors and eight slots;

[0035]FIG. 3 depicts a schematic view of an embodiment of a system thatincludes at least one magnetic lens that has at least one pole piecehaving at least two sectors;

[0036]FIG. 4 depicts a flow chart of an embodiment of a method forapplying a magnetic field to a charged particle beam using a magneticlens that includes at least one pole piece having at least two sectors;

[0037]FIG. 5 depicts a schematic view of an embodiment of an apparatusconfigured to control a magnetic field of a magnetic lens;

[0038]FIG. 6a depicts a flow chart of an embodiment of a method forcontrolling a magnetic field of a magnetic lens in which a magneticsensor is disposed within a magnetic fringe field area of a magneticlens;

[0039]FIG. 6b depicts a schematic view of an embodiment of a magneticlens in which a magnetic sensor is disposed within a cavity interposedbetween an outer pole piece of a magnetic lens and an inner pole pieceof the magnetic lens;

[0040]FIG. 6c depicts a schematic view of an embodiment of a magneticlens in which a magnetic sensor is disposed within an inner pole pieceof the magnetic lens;

[0041]FIG. 7 depicts a plot of the hysteresis of a magnetic lens using acurrent feedback control apparatus; and

[0042]FIG. 8 depicts a plot of the hysteresis of a magnetic lens using amagnetic field feedback control apparatus.

[0043] While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof are shown by way ofexample in the drawings and will herein be described in detail. Itshould be understood, however, that the drawings and detaileddescription thereto are not intended to limit the invention to theparticular form disclosed, but on the contrary, the intention is tocover all modifications, equivalents and alternatives falling within thespirit and scope of the present invention as defined by the appendedclaims.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0044] Turning now to the drawings, FIG. 1 illustrates a perspectiveview of an embodiment of a magnetic lens. As used herein, a “magneticlens” is generally defined as a magnetic circuit configured to apply amagnetic field to a charged particle beam. Magnetic lens 10 may haveouter pole piece 12 coupled to inner pole piece 14. FIG. 2 furtherillustrates a side view of an embodiment of outer pole piece 12 ofmagnetic lens 10. Further description of magnetic lens 10 and outer polepiece 12 will be made with respect to both FIGS. 1 and 2. A chargedparticle beam (not shown) may be configured to travel through themagnetic lens along axis 16 from first end 18 of magnetic lens 10 tosecond end 20 of magnetic lens 10. The charged particle beam may includean electron beam or an ion beam. First end 18 and second end 20 ofmagnetic lens 10 may form two ends of a channel through the magneticlens along axis 16 through which the charged particle beam may passduring operation. The charged particle beam may also, therefore,effectively travel through the outer pole piece along axis 16 from afirst end 22 of the outer pole piece to second end 24 of the outer polepiece. Axis 16 of outer pole piece 12 may be substantially the same asaxis 16 of magnetic lens 10.

[0045] Magnetic lens 10 may be configured to apply a magnetic field tothe charged particle beam in order to alter and/or control the path ofthe charged particle beam through the magnetic lens. As such, magneticlens 10 may be configured to operate as a magnetic objective lens thatmay be incorporated into any device that uses a charged particle beamduring operation. Examples of such devices include, but are not limitedto, scanning electron microscopes, tunneling electron microscopes,e-beam lithography tools, and focused ion beam inspection tools.Magnetic lens 10 may, however, be incorporated into any device that usesan applied magnetic field to alter the direction of a charged particlebeam.

[0046] The two pole pieces, 12 and 14, may be coupled such that anegligible air gap is formed between the two pole pieces. Minimizing theair gap between the two pole pieces may be advantageous to reduce gapdeflection flux leakage, or magnetic flux that may escape through an airgap between the pole pieces. Such air gaps may induce eddy currents inthe outer pole pieces and the lens coil. In addition, thin circularshielding rings of a magnetic material may be placed in a gap betweenthe inner pole piece and the outer pole piece of the magnetic lens tofurther minimize gap deflection flux leakage.

[0047] Outer pole piece 12 and inner pole piece 14 may be formed from amagnetically “soft” material such as ferrite, iron, holium metal, orother suitable material having a high magnetic permeability with minimumhysteresis. Inner pole piece 14 may be coupled to outer pole piece 12 byscrews 26, or other suitable connecting devices. The connecting devicemay be made of non-magnetic materials. Alternatively, the two polepieces may be connected by a physically soft material such as asilicone-based polymeric material that may flex radially to accommodatedifferential thermal expansion between the materials of the magneticlens. Connecting the pole pieces using a physically soft connectingmaterial, which is non-magnetic, may reduce fluctuations in the magneticfield of the magnetic lens as the temperature of the magnetic lensvaries due to heat generated by the magnetic lens during operation. Thevariations in temperature due to heat generated during operation mayalso be reduced by cooling the magnetic lens through evaporation coolingor by forced cooling of the lens by circulating a cooling fluidproximate the magnetic lens.

[0048] Exterior surface 28 of outer pole piece 12 may have asubstantially conical shape tapering outward from second end 24 of thepole piece. The conical shape of exterior surface 28 may beadvantageous, for example, to minimize the distance between the bore,depicted as the opening in second end 20, of magnetic lens 10 and aspecimen along a path of the charged particle beam. Therefore, thedistance that a charged particle beam travels external to the appliedmagnetic field may be reduced, and even minimized. Consequently, thecharged particle beam that impinges upon a specimen may more accuratelyreflect the characteristics of the beam within the magnetic lens.Furthermore, the conical shape of exterior surface 28 also increases theamount of space which is available for other elements coupled to asystem with the magnetic lens. In a scanning electron microscope, forexample, a stage configured to support a specimen may be positioned inthe path of the electron beam proximal second end 20 of magnetic lens10. The stage may also be tilted to provide different angles forscanning the charged particle beam across the sample. Therefore, amagnetic lens having a conical shape may reduce the restrictions on themechanical devices that may be used in a scanning electron microscopeand the restrictions to the functioning of these mechanical devices.Exterior surface 28, however, may also have a substantially cylindricalshape across outer pole piece 12. Additionally, exterior surface 28 mayalso have a partially conical shape and a partially cylindrical shape.Similarly, inner pole piece 14 may also have an exterior shape which isconical, cylindrical, or a combination of the two shapes. The exteriorshapes of both pole pieces should be designed, however, to facilitateconnection of the pole pieces while minimizing any gap which may beformed between the pole pieces.

[0049] Portion 30 of outer pole piece 12 may have a number of sectors 32and a number of slots 34. Each sector 32 may be defined as the portion30 of outer pole piece 12 disposed between lateral boundaries 36 of twoslots 34. Slots 34 may be configured to extend through substantially theentire thickness of outer pole piece 12, from exterior surface 28 ofouter pole piece 12 to an interior surface (not shown) of outer polepiece 12. Slots 34, however, may also extend partially into outer polepiece 12 from exterior surface 28. Furthermore, first portion 38 ofslots 34 may extend through substantially the entire thickness of outerpole piece 12, while second portion 40 of slots 34 may partially extendinto outer pole piece 12. As such, the second potion of slots 34 mayprovide a space for coil windings configured to encircle each sectorthrough adjacent slots.

[0050] Outer pole piece 12 may have eight sectors 32 and eight slots 34.The number of sectors on the pole piece may be larger or smaller,however, depending upon the desired performance of the magnetic lens.For example, a pole piece having only two sectors and two slots maysubstantially enhance the performance of the magnetic lens in comparisonto the performance of a magnetic lens that does not have sectors.Sectors 32 and slots 34 may also be formed in inner pole piece 14 (notshown). In this manner, outer pole piece 12, inner pole piece 14 or bothpole pieces of magnetic lens 10 may have a number of sectors and slots.Sectors 32 and slots 34 may be arranged around substantially the entireportion 30 of outer pole piece 12. Alternatively, sectors 32 and slots34 may be arranged around only a fraction of portion 30 of outer polepiece 12.

[0051] Slots 34 may be spaced evenly around outer pole piece 12. Forexample, if outer pole piece 12 has eight slots, then a slot may belocated every forty five degrees on outer pole piece 12. Alternatively,slots 34 may also have an uneven spatial arrangement around outer polepiece 12. Lateral boundaries 36 surrounding slots 34 define a laterallength 42 of each slot. In this manner, each slot 34 may havesubstantially the same lateral length 42. Lateral length 42 of each slot34 may also be substantially smaller than a lateral length 44 of sectors32. Lateral length 44 of sectors 32 may be defined as a length of eachsector 32 disposed between two slots 34, and the lateral length 44 ofeach sector 32 may also be substantially equal. Alternatively, laterallength 44 of each sector 32 may vary from sector to sector.

[0052] Slots 34 may extend vertical length 46 from second end 24 ofouter pole piece 12 across portion 30 of outer pole piece 12. Verticallength 46 of slots 34 may be larger or smaller depending on the designcharacteristics of magnetic lens 10. Each slot 34 may have substantiallythe same vertical length 46, and vertical length 46 may be substantiallyequal to vertical length 48 of each sector 42. Vertical length 48 ofsectors 32 may also be defined as a length of sector 32 extending acrossportion 30 of outer pole piece 12 from second end 24 of the pole piece.Therefore, each sector 32 may also have a vertical length 48 that may besubstantially equal to vertical length 48 of the other sectors.

[0053] A primary coil winding (as shown in FIG. 6a as primary coilwinding 134 disposed within magnetic lens 110) may also be disposedwithin magnetic lens 10 and may be interposed between outer pole piece12 and inner pole piece 14. As such, the primary coil winding may bedisposed within a cavity formed between outer pole piece 12 and innerpole piece 14. The primary coil winding may be configured to generate amagnetic potential, Φ_(o), of outer pole piece 12 relative to a magneticpotential of inner pole piece 14 when a current, I_(o), is applied tothe primary coil winding. The primary coil winding may be a number ofturns of a conductive wire. The number of turns of the conductive wiremay be approximately 100 to approximately 1000. The number of turns ofthe coil may be larger or smaller, however, depending on, for example,the intended use for the magnetic lens. The primary coil winding may beformed of an electrically conductive wire, such as copper, anodizedaluminum, or other suitable material. The primary coil winding may alsobe cooled by various means during operation in order to reduce the heatgenerated by the coil by evaporation cooling or by forced coolinginvolving circulating a cooling fluid proximate the primary windingcoil.

[0054] Magnetic lens 10 may also have a number of sector coil windings50. Each sector coil winding 50 may be coupled to one sector 32 ofmagnetic lens 10. Each sector coil winding 50 may be wound throughopening formed in outer pole piece 12 by two adjacent slots 34. As such,each sector coil winding 50 may be arranged to encircle one sector 32 ofthe magnetic lens. The sector coil winding may, therefore, encircle aportion of the sector or substantially the entire sector and may alsoinclude a number of turns of a conductive wire. The number of turns ofthe conductive wire may be configured to encircle the sector in adirection substantially perpendicular to the path of the chargedparticle beam through the magnetic lens. The number of turns for eachsector coil winding may be smaller than the number of turns for theprimary coil winding. The sector coil winding may also be formed of anelectrically conductive material and may also be cooled during operationas described above.

[0055] The magnetic potential of each sector 32 may be first establishedby the magnetic potential Φ_(o), of outer pole piece 12 generated by theprimary coil winding. Therefore, when a current is applied to theprimary coil winding, a magnetic potential of outer pole piece 12relative to inner pole piece 14 may be generated, which may be appliedapproximately equally to each sector 32 of magnetic lens 10. Current,I_(i), may also be passed through each sector coil winding 50 togenerate a magnetic potential, Φ_(i), for each sector 32 of magneticlens 10. A magnetic potential may, therefore, also be individuallygenerated on each sector 32 of magnetic lens 10. The resulting magneticpotential on the i^(th) sector, Φ_(i), may, therefore, be approximatelyequal to the sum of the magnetic potential resulting from the currentapplied to the primary coil winding, Φ_(o), and the magnetic potentialresulting from the current applied to each sector coil winding, ΔΦ_(i),or Φ_(i)=Φ_(o)+ΔΦ_(i), where ΔΦ_(i) is approximately proportional to thecurrent applied to the sector coil winding, I_(i). As such, the magneticfield which may be applied to the charged particle beam may include themagnetic potential of outer pole piece 12 relative to inner pole piece14 and a magnetic potential of each sector 32 of outer pole piece 14.

[0056] In addition, current I_(i) passed through each sector coilwinding 50 may include the zero, first, and second order harmonics whichmay be expressed by the following equation: I_(i)=A*cos (0*i)+B*cos(π/4*i+β)+C*cos (π/2*i+γ). The quantity “A” may represent the magnitudeof the fine adjustment of the focus strength of the magnetic lens whichis applied to the magnetic lens by the magnetic potential generated bythe i^(th) sector. The quantity “B” may represent the amount of magneticaxis displacement in the magnetic lens which is applied to the magneticlens by the magnetic potential generated by the i^(th) sector, and “β”may represent the direction of the magnetic axis displacement. Thequantity “C” may represent the strength of the stigmation of themagnetic lens which is applied to the magnetic lens by the magneticpotential generated by the i^(th) sector, and “γ” may represent thedirection of the stigmation axis. Therefore, the magnetic potentialgenerated on each sector of the pole piece may be used to adjust thefocus strength of the magnetic lens, the amount of magnetic axisdisplacement of the magnetic lens, and/or the strength of stigmation ofthe magnetic lens. In this manner, a current may be applied to eachsector coil winding of the magnetic lens to alter the magnetic potentialapplied to the charged particle beam.

[0057] The performance of a magnetic lens may be substantially enhancedby incorporating sectors on at least one pole piece of the magneticlens. For example, altering the magnetic potential of the magnetic lensby generating individual magnetic potentials for each sector that arecoupled to separate sector coil windings may enable the symmetry ofmagnetic lenses to be broken in a controlled fashion. The potentialadvantages of such a magnetic lens include improved capability to makefine adjustments to the focusing strength of the magnetic field.Furthermore, a sectored magnetic lens may enable displacing the magneticaxis of the magnetic lens that may be useful for the purpose of aligningthe lens axis with an off-axis charged particle beam in order tominimize aberrations in the charged particle beam. Displacing themagnetic axis may also allow any errors in the symmetry of the magneticlens to be corrected in order to obtain a symmetric magnetic field.Being able to correct the symmetry of a magnetic lens may further reducethe rejection rate of magnetic lenses associated with magnetic lensmanufacturing. Additionally, displacing the magnetic axis may be usefulin producing a deflection of the charged particle beam to affect thetrajectory of the charged particle beam. In this manner, a sectoredmagnetic lens may provide improved control over the landing position andthe landing angle of the charged particle beam on a specimen or adetector. A stigmator field may also be produced using a magnetic lenshaving sectors on at least one pole piece, which may correct for otherstigmating forces on the charged particle beam. Furthermore, because asectored magnetic lens may be operated as a multi-pole device withoutinserting additional devices within the magnetic lens, the sectoredmagnetic lens also provides the benefits of a multi-pole device whilemaintaining a co-linear path for the charged particle beam. Therefore,an advanced magnetic lens design is provided without increasing thecomplexity of the overall optical design of a system in which the lensis incorporated.

[0058] A magnetic lens having at least one pole piece which includes atleast two sectors may also be incorporated into a system configured toinspect a specimen such as a semiconductor device. Inspectingsemiconductor devices is an important step in manufacturing asemiconductor device. Inspection of semiconductor devices may usually beperformed to control and improve fabrication processes. Inspection maybe performed after individual processes have been performed or after theentire device has been fabricated. In addition to semiconductor devices,inspection of substantially transparent reticles may also be performedduring semiconductor manufacturing. Reticles may be used in lithographyto transfer a pattern to a resist on a semiconductor substrate.Therefore, a defect in or on a reticle will also be transferred to thesemiconductor device. As such, careful inspection of the reticle mayusually be performed during manufacture of the reticle itself and duringsubsequent use in semiconductor device manufacturing.

[0059] As the dimensions of devices shrink, it is becoming increasinglydifficult to successfully fabricate semiconductor devices. Therefore, itis also becoming increasingly important to monitor, control, and improvethe performance of semiconductor fabrication processes. Analysis offabrication processes, such as lithography and etch, may typically beperformed by generating an image of the device and analyzing thesemiconductor process by observing the image quality and measuringcritical dimensions of features of the device. Due to reductions infeature sizes of a semiconductor device, an inspection tool thatutilizes a charged particle beam to generate an image of a semiconductordevice may generally be used to inspect the manufactured devices.Examples of such devices include scanning electron microscopes,tunneling electron microscopes and focused ion beam inspection devices.

[0060]FIG. 3 illustrates an embodiment of a system configured to inspecta specimen. System 54 may include charged particle beam source 56, suchas an ion beam or an electron beam, configured to generate chargedparticle beam 58. Charged particle beam source 56 may be any known inthe art such as a cold field emission source or a thermal field emissionsource. System 54 may also include several positioning devices 60located along the path of charged particle beam 58 and configured todirect the charged particle beam to a focusing device. Positioningdevices 60 may include electrostatic or electromagnetic deflectors,beam-limiting apertures, Wien filters, and magnetic condenser lenses.Appropriate positioning devices, however, may vary depending upon, forexample, the intended application for the system and may include anyknown in the art. The positioning devices may be configured to alter thepath of the charged particle beam such that the charged particle beammay be substantially aligned with a first end of the magnetic lens. Inaddition, the positioning devices may also be configured to apply aninitial correction to the charged particle beam to reduce effects suchas chromatic aberrations, and/or dispersion in the energy of the chargedparticle beam.

[0061] System 54 may include at least one magnetic lens 62. Magneticlens 62 may be positioned such that charged particle beam 58 may enterthe magnetic lens at a first end of the magnetic lens and may exit themagnetic lens at a second end of the magnetic lens. A substantiallyco-linear void in the magnetic lens from the first end to the second endmay provide an appropriate path for the charged particle beam.Therefore, magnetic lens 62 may be configured to apply a magnetic fieldto charged particle beam 58 as the charged particle beam travels throughthe magnetic lens. At least one pole piece of magnetic lens 62 may haveat least two sectors and at least two slots. The sectors may be disposedbetween lateral boundaries of the slots in the pole piece. Magnetic lens62 may have a primary coil winding disposed within the magnetic lens.The magnetic lens may also include a number of secondary coil windings,and each sector coil winding may be coupled to one sector of the polepiece. As such, the primary coil winding may be configured to generate amagnetic potential of the outer pole piece relative to the inner polepiece, and each secondary coil winding may be configured to generate amagnetic potential of one sector of the pole piece. The magnetic fieldapplied to charged particle beam 58 by magnetic lens 62, therefore, mayinclude the magnetic potential of the outer pole piece relative to theinner pole piece and the magnetic potential of at least one sector ofthe pole piece. The magnetic lens may be further configured as describedherein. The magnetic lens may be also configured to operate as amagnetic objective lens to focus the charged particle beam onto asemiconductor device. Focusing the charged particle beam may includereducing aberrations in the charged particle beam and reducing thediameter of the charged particle beam to a spot size which isappropriate for imaging a semiconductor device.

[0062] System 54 may further include stage 64 configured to support atleast a portion of specimen 66, which may include at least a portion ofa semiconductor device formed on a semiconductor substrate, or a productwafer. Stage 64 may be positioned along the path of charged particlebeam 58. The stage may be any mechanical device known in the art. Forexample, stage 64 may include a holder configured to engage a stud. Thestud may include a horizontally flat upper surface configured to supportthe specimen and a post that may be substantially vertical to the uppersurface. Semiconductor device 66 may be attached to a stud using anadhesive. The holder may be positioned in the path of the chargedparticle beam by additional mechanical devices and may include voidsconfigured to engage the post of the stud. As such, the stud may be alsobe positioned in the path of charged particle beam when the stud isengaged within the holder. Alternatively, stage 64 may have a flatsurface which may support specimen 66. The flat surface of the stage mayalso have small holes through which a vacuum source may be connected. Inthis manner, the stage may be configured to engage the specimen using avacuum which may be generated by the vacuum source during operation. Thestage may also be positioned in the path of the charged particle beammanually or by a mechanical device controlled by a controller computer.

[0063] Specimen 66 may also be positioned in the path of chargedparticle beam 58 such that the charged particle beam may interact withthe specimen to generate a secondary beam of charged particles 68.Secondary beam of charged particles 68 may include secondary chargedparticles, which may emanate from recesses of the specimen,back-scattered charged particles, which may emanate from the surface ofthe specimen, and transmitted electrons, which may pass through thespecimen, such as a substantially transparent reticle. System 54 mayalso include detector 70 or a plurality of detectors and at least onedevice 72 configured to direct the secondary beam of electrons to thedetectors. Device 72 may include, for example, a Wien filter. The Wienfilter, or other suitable device, may be configured to alter the path ofthe secondary electrons without affecting the path of the chargedparticle beam which is being directed to the semiconductor device.Detector 70 may be a Schottky solid state barrier detector, or othersuitable detectors, and may also be coupled to operating system 74,which may be incorporated into the system. Operating system 74 may beconfigured to receive a signal from detector 70, to analyze the signal,and to generate information about characteristics of specimen 66 such asfeature size. Operating system 74 may also be coupled to imaging device76, which may be a cathode ray tube. In this manner, system 54 may alsobe configured to generate image profile characteristics of specimen 66.The characteristics of the device may be used to control and/or alter aprocess, which was used to fabricate the specimen. Additional featuresand devices that may also be incorporated into the system areillustrated, for example, in U.S. Pat. No. 4,928,010 to Saito et al.,U.S. Pat. No. 5,241,176 to Yonezawa, U.S. Pat. No. 5,502,306 toMeisburger et al., U.S. Pat. No. 5,578,821 to Meisburger et al., U.S.Pat. No. 5,665,968 to Meisburger et al., U.S. Pat. No. 5,717,204 toMeisburger et al., U.S. Pat. No. 5,869,833 to Richardson et al., U.S.Pat. No. 5,872,358 to Todokora et al., and U.S. Pat. No. 5,973,323 toAdler et al. and are incorporated by reference as if fully set forthherein.

[0064]FIG. 4 illustrates an embodiment of a method for applying amagnetic field to a charged particle beam. As shown in step 78, acharged particle beam may be generated by supplying power to a chargedparticle beam source. The charged particle beam source may be any knownin the art including a cold field emission source or a thermal fieldemission source. Additionally, the charged particle beam may include abeam of electrons or ions. The charged particle beam source may bedisposed within an optical column, which may be held under vacuumconditions during operation. As shown in step 80, the charged particlebeam may be directed to a first end of the magnetic lens by applyingelectrostatic or magnetic fields to the charged particle beam. Forexample, prior to traveling through the magnetic lens, the chargedparticle beam may be passed through several positioning devices locatedalong the path of the charged particle beam. The positioning devices andthe magnetic lens may also be incorporated into the optical columncontaining the charged particle beam source. As such, the positioningdevices and the magnetic lens may be operated while the optical columnis under vacuum conditions. Positioning devices may includeelectrostatic or electromagnetic deflectors, beam-limiting apertures,Wien filters and magnetic condenser lenses. The positioning devices mayalter the path of the charged particle beam to substantially align thecharged particle beam with a first end of the magnetic lens. Inaddition, the positioning device may also apply an initial correction tothe charged particle beam to reduce effects such as chromaticaberrations, or dispersion in the energy of the charged particle beam.

[0065] The charged particle beam may be directed to the magnetic lenssuch that the charged particle beam may travel substantially through themagnetic lens which includes at least one pole piece. The chargedparticle beam may enter the magnetic lens at a first end of the magneticlens and may exit the magnetic lens at a second end of the magneticlens. The magnetic lens may be configured as described herein.

[0066] As shown in step 82, a magnetic field may be generated within themagnetic lens by driving a magnetic potential of the pole piece. Forexample, a first current may be applied to a primary coil windingcoupled to the magnetic lens. A second current may also be applied to atleast one sector coil winding coupled to a sector of the magnetic lens.In this manner, a magnetic potential of at least one sector of themagnetic lens may also be generated. Therefore, the magnetic fieldapplied to the charged particle beam traveling through the magnetic lensmay include the magnetic potential of the outer pole piece relative tothe inner pole piece and the magnetic potential of at least one sector.The magnetic potential of at least one sector of the pole piece mayalter the focus strength of the magnetic lens, the magnetic axisdisplacement of the magnetic lens, and/or the strength of stigmation ofthe magnetic lens. In this manner, the magnetic lens may have anenhanced capability to reduce aberrations in the charged particle beam.

[0067] In an additional embodiment, the method, as shown in FIG. 4, mayinclude additional steps to focus a charged particle beam on a specimen.The specimen may include a semiconductor device, a feature or a level ofa semiconductor device, or other suitable specimen such as asubstantially transparent reticle configured to transfer a pattern to aresist in lithography, a photoresist layer suitable for e-beamlithography, or a biological sample. The specimen may also be an entiresemiconductor substrate that has been processed using a semiconductorfabrication process or a portion of the semiconductor substrate. Forexample, a portion of a semiconductor substrate may be formed by cuttingthe substrate into segments at appropriate positions on the substrate.As such, a cross-sectional portion of semiconductor device, or othersemiconductor feature of interest, may be exposed to the chargedparticle beam. An appropriate portion of the specimen may also be largeror smaller depending on the capability of the device coupled to thecharged particle beam. As shown in step 84, a specimen such as asemiconductor device may be fabricated on a semiconductor substrateusing a semiconductor manufacturing process. The semiconductormanufacturing process may be any process known in the art ofsemiconductor manufacturing, such as lithography, etch, ionimplantation, deposition, chemical mechanical polishing, and/or plating.

[0068] At least a portion of the specimen may be positioned in the pathof the charged particle beam prior to generating the charged particlebeam, as shown in step 86. The specimen may be positioned in the path ofthe charged particle beam by placing the specimen on a stage. The stagemay be located substantially within the optical column or proximal theoptical column such that the stage and the specimen are also undervacuum conditions during operation. The stage may be configured tosupport the specimen such that a position of the specimen in the path ofthe charged particle beam may be maintained throughout a process. Forexample, the specimen may be attached to a stud using an adhesive. Thestud may be configured as described herein. Alternatively, the stage mayhave a flat surface configured to support substantially the entirespecimen such as a semiconductor substrate or product wafer. The flatsurface of the stage may also have holes through which a vacuum may bepulled in order to retain the position of the specimen in the path ofthe charged particle beam. The stage, however, may also be any suitablemechanical device known in the art. The stage may also be positioned inthe path of the charged particle beam manually or by a mechanical devicethat may be controlled by a controller computer.

[0069] In an additional embodiment, the method, as shown in FIG. 4, mayinclude additional steps to detect at least one secondary beam ofcharged particles, as shown in step 88. The secondary beam of chargedparticles may be produced as a result of interactions between thecharged particle beam and the specimen. The secondary beam of chargedparticles may include secondary charged particles, back-scatteredcharged particles, and/or transmitted charged particles. The secondarybeam of electrons may be directed back through the magnetic lens. Adevice such as a Wien filter, which may generate an electrostatic fieldor a combination of electrostatic and magnetic fields, may be used todirect the secondary beam of charged particles to a detector or aplurality of detectors. As such, the Wien filter direct the secondarybeam to a detector at a high angle of incidence from the original pathof the secondary beam. The Wien filter, however, may apply the field tothe secondary beam without affecting the trajectory of the chargedparticle beam, which may also pass through the Wien filter. Thesecondary beam of charged particles may then be detected by a Schottkysolid state barrier detector, or other suitable detector. The detectormay be configured to detect the secondary beam of charged particles andto generate a signal responsive to the secondary beam.

[0070] In an embodiment, an image of the specimen may be generatedsubsequent to detecting the secondary beam of charged particles, asshown in step 90. An operating system coupled to the detector mayreceive a signal generated by at least one detector. The signal from thedetector may be responsive to the secondary beam of charged particles.The operating system may include a computer such as a personal computeror a mainframe computer, which may have an appropriate software packageto perform an appropriate set of operations on the signal from thedetector. As such, the operating system may analyze the signal from thedetector and generate an output signal representative of an image of thespecimen. An imaging system such as a cathode ray tube, which may becoupled to the operating system, may receive the output signal from theoperating system and generate an image of the specimen. The image may beused to analyze physical characteristics of the specimen such as featuresize and vertical profile quality. The physical characteristics of thedevice may then also be used to control and/or improve the a processwhich was used to fabricate the specimen.

[0071] In a further embodiment, the method as illustrated in FIG. 4 mayalso include applying a coarse focus adjustment to the charged particlebeam, as shown in step 92. An initial magnetic potential may begenerated within the magnetic lens by applying a current to a primarycoil winding of the magnetic lens. The initial magnetic potential mayalso be generated within the magnetic lens by further applying a currentto sector coil winding coupled to a sector of the magnetic lens. Theinitial magnetic potential may be a function of the magnetic potentialgenerated by the primary coil winding and the magnetic potentialgenerated by the sector coil winding as described above. The initialmagnetic potential may adequately focus, or reduce aberrations in, thecharged particle beam on a specimen in order to produce an image of thespecimen. The image of the specimen may be generated as described in theabove embodiments. For example, the image of the specimen may begenerated by detecting at least one secondary beam of charged particleswhich may be result from interactions between the charged particle beamand the specimen. The image of the specimen may be observed using aimaging device such as a cathode ray tube.

[0072] The clarity of the image of the specimen may generally bedependent on the aberrations such as chromatic aberration and stigmationin the charged particle beam, which interacts with the specimen.Therefore, the initial magnetic potential may not sufficiently reduceaberrations in, or focus, the charged particle beam in order to obtainan adequate image of the specimen. In step 92, a coarse focus adjustmentmay be applied to the charged particle beam by altering the magneticpotential within the magnetic lens. The coarse focus adjustment mayreduce aberrations in the charged particle beam, which are presentdespite the initial magnetic field. Altering the magnetic potentialwithin the magnetic lens may include altering and/or controlling thecurrent to the primary coil winding and the current to at least onesector coil winding using a manually-controlled device or a controllercomputer. The magnetic potential within the magnetic lens may also bealtered by altering and/or controlling the current being applied toseveral sector coil windings using a manually-controlled device or acontroller computer.

[0073] The manually-controlled device and the controller computer mayboth be coupled to the magnetic lens. For example, an operator mayobserve the image of the specimen generated by the initial magneticpotential of the magnetic field and may manually adjust the currentbeing applied to the primary coil winding and the current being appliedto at least one of the sector coil windings by using amanually-controlled device. The manually-controlled device may include adial, or other suitable device, which may be coupled to the power supplyof the primary coil winding or a sector coil winding. Alternatively, acontroller computer may analyze a gray-scale image of the specimengenerated by the initial magnetic potential of the magnetic field. Thecontroller computer may alter the primary current applied to the primarycoil winding and the current applied to at least one sector coil windingusing a set of predefined mathematical equations.

[0074] After the coarse focus adjustment, the current being applied tothe primary coil winding may generally remain constant. For example, thecoarse focus adjustment may adequately reduce aberrations in the chargedparticle beam such that only fine adjustments to the charged particlebeam may be required. Additionally, maintaining a constant current tothe primary coil winding may simplify further operation of the magneticlens. In step 94, a fine focus adjustment may also be applied to thecharged particle beam by further altering the magnetic potential withinthe magnetic lens. Further altering the magnetic potential within themagnetic lens may include altering and controlling the current to atleast one sector coil winding, or multiple sector coil windings, using amanually-controlled device or a controller computer. Themanually-controlled device and the controller computer may both becoupled to the magnetic lens. As such, further altering of the magneticpotential within the magnetic lens may be performed using themanually-controlled device or the controller computer, as described instep 92. For example, an operator may manually adjust the current beingapplied to at least one of the sector coil windings by using amanually-controlled device. As described above, the manually-controlleddevice may include a dial, or other suitable device, which may becoupled to the power supply of at least one sector coil winding.Additional dials, or other suitable devices, may also be coupled toadditional sector coil windings such that the current being applied toeach sector coil winding may be controlled individually. Alternatively,a controller computer may automatically adjust the current being appliedto at least one sector coil winding by using a set of predefinedmathematical equations. The fine focus adjustment to the chargedparticle beam may include an adjustment to the magnitude of a focusstrength of the magnetic lens, an adjustment to an amount of magneticaxis displacement of the magnetic lens, and/or an adjustment to astrength of stigmation of the magnetic lens. The fine focus adjustmentmay, therefore, generate a magnetic potential within the magnetic lensto reduce aberrations in the charged particle beam such that an adequateimage of the specimen may be generated.

[0075] Furthermore, the methods for applying a magnetic field to acharged particle beam may be integrated into a controller for a magneticlens. The controller may by a computer system configured to operatesoftware to control the operation of the magnetic lens. The computersystem may include a memory medium on which computer programs foroperating the magnetic lens and performing calculations related to thedata collected. The term “memory medium” is intended to include aninstallation medium, e.g., a CD-ROM, or floppy disks, a computer systemmemory such as DRAM, SRAM, EDO RAM, Rambus RAM, etc., or a non-volatilememory such as a magnetic media, e.g., a hard drive, or optical storage.The memory medium may include other types of memory as well, orcombinations thereof. In addition, the memory medium may be located in afirst computer in which the programs are executed, or may be located ina second different computer that connects to the first computer over anetwork. In the latter instance, the second computer provides theprogram instructions to the first computer for execution. Also, thecomputer system may take various forms, including a personal computersystem, mainframe computer system, workstation, network appliance,Internet appliance, personal digital assistant (PDA), television systemor other device. In general, the term “computer system” may be broadlydefined to encompass any device having a processor that executesinstructions from a memory medium.

[0076] The memory medium preferably stores a software program for theoperation of the magnetic lens. The software program may be implementedin any of various ways, including procedure-based techniques,component-based techniques, and/or object-oriented techniques, amongothers. For example, the software program may be implemented usingActiveX controls, C++ objects, JavaBeans, Microsoft Foundation Classes(MFC), or other technologies or methodologies, as desired. A CPU, suchas the host CPU, executing code and data from the memory medium includesa device configured to create and execute the software program accordingto the methods described above.

[0077] Various embodiments further include receiving or storinginstructions and/or data implemented in accordance with the foregoingdescription upon a carrier medium. Suitable carrier media include memorymedia or storage media such as magnetic or optical media, e.g., a diskor CD-ROM, as well as signals such as electrical, electromagnetic, ordigital signals, conveyed via a communication medium such as networksand/or a wireless link.

[0078] The software for the magnetic lens may be used to control themagnetic field applied to a charged particle beam. Preferably,predefined mathematical equations that describe the relationshipsbetween the magnetic potentials of the pole piece, the sectors of thepole piece and the current applied to each coil winding of the magneticlens may be incorporated into the software. The software may beconfigured to measure a magnetic field generated within a magnetic lens.The magnetic lens may be configured as described herein. The softwaremay also be configured to determine a primary current in response to themeasured magnetic field, which may be applied to a primary coil windingcoupled to a pole piece of the magnetic lens. The software may befurther configured to determine a secondary current in response to themeasured magnetic field, which may be applied to at least one secondarycoil winding coupled to a sector of the outer pole piece. Additionally,the software may be configured to control the applied primary currentand the applied secondary current. In this manner, the software may beconfigured to maintain a predetermined magnetic field within themagnetic lens or to correct for magnetic field drift within the magneticfield of the magnetic lens. Magnetic field drift may occur due to theperformance of the electron beam source, hysteresis effects within themagnetic lens, temperature dependent properties of the magnetic lens,and/or drift caused by sources external to the magnetic lens.

[0079]FIG. 5 illustrates an embodiment of an apparatus configured tocontrol a magnetic field strength, or the magnetic flux density, of amagnetic lens. Apparatus 96 may include magnetic sensor 98 that may bedisposed within a magnetic field generated by magnetic lens 100.Apparatus 96 may also include control circuit 102 which may be coupledto magnetic sensor 98 and magnetic lens 100. The apparatus may beconfigured to continuously control the magnetic field strength of themagnetic lens. Alternatively, the apparatus may be configured tointermittently control the magnetic field strength of the magnetic lens.For example, the apparatus may be configured to alter and/or control themagnetic field strength of the magnetic lens approximately once persecond. The magnetic lens may be configured and used as describedherein. For example, magnetic lens 100 may be configured to apply amagnetic field to a charged particle beam, and the charged particle beammay be configured to travel through the magnetic lens. Magnetic lens 100may also be configured to operate as a magnetic condenser lens or amagnetic objective lens. Therefore, the magnetic lens may be configuredto reduce aberrations in a charged particle beam, such as chromaticaberration and stigmation. The apparatus, however, may also be coupledto any magnetic lens configured to generate a magnetic field when acurrent is applied to the magnetic lens.

[0080] Apparatus 96 may be coupled to a magnetic lens and to a system,which may utilize the magnetic lens during use, such as a scanningelectron microscope, a tunneling electron microscope, a focused ion beamdevice, or any other system which may be configured to inspect orfabricate a specimen such as a semiconductor device using a chargedparticle beam. The semiconductor device may be fabricated, prior toinspection, using a semiconductor manufacturing process, such aslithography, etch, ion implantation, deposition, chemical-mechanicalpolishing or plating. Additionally, the semiconductor device may be aportion of a device that may be formed on a semiconductor substrate.Alternatively, the semiconductor device may be a working device whichmay be formed on a semiconductor substrate during semiconductormanufacturing. The magnetic lens, however, may also be coupled to anysystem which utilizes a charged particle beam during operation such asan e-beam lithography system.

[0081] The system may include at least one magnetic lens. The magneticlens may be configured to apply a magnetic field to a charged particlebeam. The system may also include a charged particle beam sourceconfigured to produce the charged particle beam. The system may befurther configured such that the charged particle beam may be configuredto travel through the magnetic lens prior to interacting with thespecimen. In addition, the system may include a stage configured tosupport at least a portion of the specimen. The stage may also bepositioned along the path of the charged particle beam such that thecharged particle beam may interact with the specimen. The system mayalso include any of the devices as described and shown in FIG. 3including, but not limited to, electrostatic or electromagneticdeflectors, beam-limiting apertures, Wien filters, magnetic condenserlenses, Schottky solid state barrier detectors, an operating system, andan imaging device.

[0082] Magnetic sensor 98 may be configured to generate an output signalresponsive to a magnetic field strength of magnetic lens 100. Any signalthat may be responsive to a magnetic field strength or another conditionor property of the magnetic lens may be a mathematical representation ofthe magnetic field strength or another condition or property of themagnetic lens. For example, a signal may be linearly, proportionally,inversely, or logarithmically related to a condition or property of themagnetic lens. The magnetic sensor may be configured to operate as aHall-effect sensor and may provide an output signal (e.g., a voltage).The output signal may be linearly and proportionally related to themagnetic field strength within the magnetic lens. The output signal ofmagnetic sensor 98 may be an analog signal or a digital signal. Examplesof suitable Hall-effect sensors may be commercially available fromAllegro Microsystems, Inc. of Worcester, Mass. Other suitable magneticsensors, however, may also be used, which provide an output signalresponsive to the magnetic field strength of the magnetic lens. Themagnetic sensor may be disposed within a magnetic fringe field area ofthe magnetic lens, as shown in FIG. 5. Alternatively, the magneticsensor may be disposed within a cavity of the magnetic lens. The cavitymay be interposed between an inner pole piece of the magnetic lens andan outer pole piece of the magnetic lens. Additionally, the cavity maybe disposed within the inner pole piece of the magnetic lens.

[0083] The apparatus may also include a temperature sensor to measuretemperature variations of the magnetic lens. In an embodiment, apparatus96 may include temperature sensor 104 coupled to the magnetic lens andthe magnetic sensor. Temperature sensor 104 may be a matchedtemperature-dependent circuit element. Other suitable temperaturesensors, however, may also be included in the apparatus. Temperaturesensor 104 may be configured to monitor the temperature of the magneticlens and to generate an output signal, or a temperature signalresponsive to a temperature of the magnetic lens. Therefore, magneticsensor 98 may be further configured to receive the temperature signalfrom temperature sensor 104. In this manner, temperature sensor 104 maybe positioned proximal magnetic sensor 98 such that magnetic sensor 98and temperature sensor 104 may be exposed to an approximately equaltemperature of the magnetic lens 100. Furthermore, magnetic sensor 98may be further configured to generate an output signal responsive to themagnetic field strength of the magnetic lens in addition to thetemperature of the magnetic lens. As such, the output signal of magneticsensor 98 may be altered to include variations in the temperature of themagnetic lens.

[0084] Control circuit 102 coupled to the magnetic sensor may beconfigured to receive the output signal from magnetic sensor 98. Thecontrol circuit may also include a low-pass filter element (not shown)configured to receive the output signal from the magnetic sensor. Thelow-pass filter element may also be configured to reduce fluctuations inthe output signal from magnetic sensor 98. Therefore, the low-passfilter element may prevent fluctuations in the output signal from themagnetic sensor from being transferred into output signals generated bythe control circuit. The control signal may be configured to generate anoutput signal in response to the output signal from the magnetic sensor.The signals generated by the control circuit may be used to control themagnetic lens. As such, the low-pass filter element may preventfluctuations in the current supplied to the magnetic lens, which mayadversely affect the performance of the magnetic lens.

[0085] Control circuit 102 may also be configured to receive inputsignal 106, which may be responsive to a predetermined magnetic fieldstrength. An operating system (not shown) may be coupled to theapparatus and may be configured to generate input signal 106. Theoperating system may be manually-controlled or computer-controlled.Input signal 106 may include a voltage, which may have a linear andproportional relationship to the predetermined magnetic field strengthof the magnetic lens. The predetermined magnetic field strength may be avariable magnetic field strength or a constant magnetic field strength.Therefore, the predetermined magnetic field strength may vary inresponse to a desired performance of the magnetic lens, which may alsovary over time. In addition, the predetermined magnetic field strengthmay be constant in response to a desired performance of the magneticlens, which may be sustained over a period of time. In this manner, thecontrol circuit may be configured to alter the magnetic field strengthof the magnetic lens or to maintain the magnetic field strength of themagnetic lens.

[0086] The predetermined magnetic field strength may be determined by anoperator. For example, the manually-controlled operating system may beconfigured to provide information, which may describe the performance ofthe magnetic lens, to an operator. The operator may determine thedesired performance or function of the magnetic lens by analyzing theinformation about the performance of the magnetic lens. Themanually-controlled operating system may be further configured toreceive an input signal from the operator representative of a desiredperformance or function of the magnetic lens. The input from theoperator may also be representative of a predetermined magnetic fieldstrength, which may enable the desired performance of the magnetic lens.The manually-controlled operating system may, therefore, be configuredto then convert the input from the operator to an input signal, whichmay be received by the control circuit.

[0087] Alternatively, the predetermined magnetic field strength may bedetermined by a computer-controlled operating system. Thecomputer-controlled operating system may include a computer such as apersonal computer or a mainframe computer. For example, thecomputer-controlled operating system may be configured to receiveinformation which may be descriptive of the performance of the magneticlens. The computer-controlled operating system may be further configuredto analyze the information and to generate an input signalrepresentative of a desired performance or function of the magneticlens. The input from the computer-controlled operating system may,therefore, be representative of the differences between the desiredperformance of the magnetic lens and the current performance of themagnetic lens. As such, the input signal from the computer-controlledoperating system may also be representative of a predetermined magneticfield strength, which may enable the desired performance or the desiredfunction of the magnetic lens.

[0088] Control circuit 96 may also be configured to generate a controlsignal responsive to the output signal from magnetic sensor 98 and inputsignal 106. In addition, control circuit 96 may be configured togenerate a control signal responsive to the output signal and the inputsignal. The control circuit may also include operational amplifier 108configured to receive the output signal from magnetic sensor 98 andinput signal 106. The operational amplifier may be further configured togenerate a comparison signal, which may be responsive to differencesbetween the output signal from the magnetic sensor and the input signalfrom the operating system. For example, the operational amplifier mayperform any number of comparisons between the output signal and theinput signal including, but not limited to, subtraction, multiplication,division, and algorithms. Operational amplifier 108 may also beconfigured to generate a control signal, which may be a function of thecomparison signal. For example, the operational amplifier may beconfigured to compare the output signal from the magnetic sensor and theinput signal and to apply a gain to a difference between the twosignals. Alternatively, the control circuit may include any circuitelement or a plurality of circuit elements, which may be configured toperform the operations described herein.

[0089] Control circuit 96 may also be configured to drive magnetic lens100 by applying a current to at least one coil of the magnetic lens. Thecurrent may be responsive to the control signal, which may be generatedby the control circuit. The applied current may be effective to generatea magnetic field strength within the magnetic lens that is closer to thepredetermined magnetic field strength than the measured magnetic fieldstrength. Alternatively, the applied current may be effective togenerate a magnetic field strength within the magnetic lens that issubstantially equal to the predetermined magnetic field strength.Control circuit 96 may also include an electronic current drive system(not shown), which may be configured to receive the control signal fromthe control circuit and to drive the magnetic lens by applying a currentto at least one coil of the magnetic lens. As such, the electroniccurrent drive system may receive the control signal from operationalamplifier 108, which may be included in control circuit 96. In addition,the electronic current drive system may be configured to perform anoperation on the control signal. For example, the electronic currentdrive system may be configured to convert the control signal from ananalog signal to a digital signal. The electronic current drive systemmay be further configured to alter and control a power source inresponse to the control signal. The power source may be configured tosupply a current to at least one coil of the magnetic lens. Therefore,the current, which may be supplied to the magnetic lens, may be alteredand/or controlled by the control circuit.

[0090]FIG. 6a illustrates an embodiment of a method for controlling amagnetic field strength of a magnetic lens. The method may be used tocontinuously or to intermittently control the magnetic field strength ofa magnetic lens. Prior to performing the method, a magnetic field may begenerated within magnetic lens 110 by applying a current to at least onecoil winding such as primary coil winding 134 of the magnetic lens. Thecurrent may be a current applied to magnetic lens 110 prior to beginningthe method such as a current appropriate for starting up a magnetic lensor a system in which the magnetic lens is used. The current may also bea current applied to magnetic lens 110 prior to beginning the methodsuch as a current appropriate for a prior use of the magnetic lens. Asshown in step 112, the method may include generating an output signalresponsive to the magnetic field strength of magnetic lens 110. Theoutput signal may be generated by using magnetic sensor 114. The outputsignal generated by the magnetic sensor may be a voltage, which may havea linear relationship to the magnetic field strength of the magneticlens.

[0091] Magnetic sensor 114 may be disposed within a magnetic field ofmagnetic lens 110. For example, magnetic sensor 114 may be disposedwithin a magnetic fringe field area of magnetic lens 110, as shown inFIG. 6a. Alternatively, magnetic sensor 114 may be disposed within acavity of the magnetic lens. The cavity may be defined as a spacebetween an outer pole piece of the magnetic lens and an inner pole pieceof the magnetic lens. As shown in FIG. 6b, magnetic sensor 114 isdisposed between an outer pole piece and an inner pole piece of magneticlens 110. In addition, magnetic sensor 114 may be disposed within aninner pole piece of the magnetic lens. As shown in FIG. 6c, magneticsensor 114 is disposed within an inner pole piece of magnetic lens 110.In this manner, magnetic sensors may be located at different positionsinternal and external to the magnetic lens.

[0092] In an embodiment, the method may include generating an outputsignal which may be responsive to a temperature of the magnetic lens, asshown in step 116. The temperature signal may be generating by usingtemperature sensor 118. The generated temperature signal may beresponsive to a temperature of magnetic lens 110. The temperature sensormay be coupled to the magnetic lens and the magnetic sensor.Furthermore, the temperature sensor may send the generated temperaturesignal to the magnetic sensor. As a result, the magnetic sensor mayalter the output signal of the magnetic sensor in response to thetemperature signal. For example, the temperature signal may be used toalter the output signal of the magnetic sensor such that the magneticsensor may be sensitive to fluctuations in the temperature of themagnetic lens. As such, temperature sensor 118 may be positionedproximal magnetic sensor 114. In this manner, the temperature signalgenerated by temperature sensor 118 may be responsive to a temperatureof magnetic lens 110 proximal magnetic sensor 114 and temperature sensor118. For example, as shown in FIG. 6a, temperature sensor 118 andmagnetic sensor 114 may be placed within a magnetic fringe field area ofmagnetic lens 110. Alternatively, as shown in FIG. 6b, temperaturesensor 118 and magnetic sensor 114 may be disposed within a cavity ofthe magnetic lens. In addition, as shown in FIG. 6c, temperature sensor118 and magnetic sensor 114 may be disposed within an inner pole pieceof magnetic lens 110.

[0093] In an additional embodiment, the method may include reducingfluctuations in the output signal, as shown in step 120. The outputsignal may be sent to a low-pass circuit element, which may be coupledto a control circuit. Reducing fluctuations in the output signal mayprovide improved control on the magnetic lens by eliminating erraticchanges in the current being applied to at least one coil of themagnetic lens. In addition, the method may include sending the outputsignal to a control circuit, as shown in step 122. The control circuitmay be coupled to the magnetic sensor and the magnetic lens.

[0094] In an embodiment, the method may include sending an input signalto the control circuit, as shown in step 124. The input signal may beresponsive to a function of a predetermined magnetic field strength. Forexample, the predetermined magnetic field strength may be representativeof a desired function or operating characteristic of the magnetic lens.Alternatively, the predetermined magnetic field strength may also be asubstantially constant magnetic field strength. In this manner, themethod may be used to eliminate variations in the magnetic fieldstrength of the magnetic lens over time. The input signal may be avoltage, which may have a linear relationship to the predeterminedmagnetic field strength of the magnetic lens. The input signal may begenerated by using an operating system, which may be coupled to thecontrol circuit. The operating system may be manually-controlled orcomputer-controlled. As such, the input signal which may be determinedby an operator or by a controller computer.

[0095] As shown in step 126, the method for controlling the magneticfield strength of a magnetic lens may include generating a controlsignal. The control signal may be responsive to a function of the outputsignal and the input signal. In an embodiment, an operational amplifiermay be coupled to the control circuit. Therefore, generating the controlsignal may include generating a comparison signal by using theoperational amplifier. The operational amplifier may be configured togenerate the control signal by comparing the output signal from themagnetic sensor and the input signal. As such, the operational amplifiermay generate a comparison signal, which may be responsive to differencesbetween the output signal and the input signal. The operationalamplifier may also be configured to perform a function on the generatedcomparison signal. For example, the operational amplifier may generatethe control signal by applying a gain to the comparison signal.

[0096] In an embodiment, the method may also include sending the controlsignal to an electronic current drive system, as shown in step 128. Theelectronic current drive system may be coupled to the control circuit.The electronic current drive system may be used to control the current,which may be applied to at least one coil of the magnetic lens. In anembodiment, as shown in step 130, the method may include applying acurrent to at least one coil of the magnetic lens. The current may beresponsive to the control signal which may be generated by the controlcircuit. Therefore, as shown in step 132, the predetermined magneticfield strength may be generated within the magnetic lens. For example,the current may be effective to generate a magnetic field strengthwithin the magnetic lens that is closer to the predetermined magneticfield strength than a first or measured magnetic field strength. Inaddition, the current may also be effective to generate a magnetic fieldstrength within the magnetic lens that is substantially equal to thepredetermined magnetic field strength.

[0097] In an embodiment, the method for controlling a magnetic fieldstrength of a magnetic lens may also include directing a chargedparticle beam through the magnetic lens. The charged particle beam maybe an electron beam or an ion beam. In this manner, the magnetic lensmay be used to apply a magnetic field to the charged particle beam. Assuch, the magnetic lens may be coupled to a device, which may use amagnetic lens to alter the path of a charged particle beam. Examples ofsuch devices may include, but are not limited to, scanning electronmicroscopes, tunneling electron microscopes, e-beam lithography devicesor focused ion beam devices. For example, the magnetic lens may beconfigured to operate as a magnetic lens incorporated into a scanningelectron microscope.

[0098] Inspecting a specimen such as a semiconductor device, which maybe formed on a semiconductor substrate, subsequent to intermediate andfinal processing steps has become an integral part of successfullyproducing working semiconductor devices. The semiconductor device may befabricated prior to inspection by using a semiconductor manufacturingprocess. The semiconductor manufacturing process may include a number ofprocessing steps as described herein. The semiconductor device may alsoinclude a semiconductor topography, which may be only a portion of aworking semiconductor device. For example, a semiconductor device may beinspected subsequent to each of the many processing steps required tofabricate a working semiconductor device.

[0099] In an embodiment, the method for inspecting a specimen mayinclude generating a magnetic field within the magnetic lens by applyinga current to at least one coil of the magnetic lens. In addition, themethod may also include applying the generated magnetic field to acharged particle beam by directing the charged particle beam through themagnetic lens. The charged particle beam may be an electron beam or anion beam. The method may also include generating the charged particlebeam by using a charged particle beam source. Prior to directing thecharged particle beam through the magnetic lens, at least a portion of aspecimen may be positioned on a stage configured to support thespecimen. The stage may be located along a path of the charged particlebeam. As such, the charged particle beam may impinge upon the specimenafter passing through the magnetic lens. The charged particle beamexiting the magnetic lens may be substantially free of aberrations.Therefore, the magnetic lens may be coupled to a scanning electronmicroscope or other suitable device, which may apply a magnetic field toa charged particle beam to alter a path of the charged particle beam, orto reduce aberrations that may be present in the charged particle beam.

[0100] Inspecting a semiconductor device may also include monitoring andaltering a magnetic field strength of the magnetic lens. Monitoring andaltering the magnetic field strength of the magnetic lens may includegenerating an output signal, which may be responsive to a magnetic fieldstrength of the magnetic lens by using a magnetic sensor. The magneticsensor may be disposed within a magnetic field of the magnetic lens. Themethod may also include sending the output signal to a control circuit,which may be coupled to the magnetic sensor and the magnetic lens. Inaddition, the method may include sending an input signal to the controlcircuit. The input signal may be responsive to a function of apredetermined magnetic field strength. The method may also include usingthe control circuit to generate a control signal which may be responsiveto a function of the output signal and the input signal. Furthermore,the method may include generating the predetermined magnetic fieldstrength within the magnetic lens by applying a current to at least onecoil of the magnetic lens. The current may be responsive to the controlsignal. The method may also include additional steps as describedherein.

[0101] The apparatus and methods for using the apparatus described abovemay provide accurate control of the magnetic field within acurrent-driven magnetic lens. The magnetic lens may be coupled to anydevice which uses a charged particle beam to perform a function. Usingthe apparatus and methods to control a magnetic field of a magnetic lensmay eliminate adverse effects of hysteresis on the performance of themagnetic lens. In addition, the apparatus and methods may also reducethe effects of temperature dependent material properties, drift incurrent drive electronics, low frequency noise, eddy currents, undesiredsuperimposed fields on the magnetic field generated by a magnetic lens.Furthermore, the apparatus and methods described above may also be usedto eliminate drift in the magnetic field strength over time from othercauses. In the application to charged particle beam devices, thisfunctionality may be useful for reproducibly tuning magnetic componentsas magnetic lenses, Wien filters, or deflection coils. Specifically, themagnetic field sensor may permit easier manual and automated operationof scanning electron microscopes and other electron beam devices. Themagnetic feedback concept may greatly increase tool stability and toolto tool consistency. Specifically, optimized values of magnetic fieldfor deflection coils, Wien filters and magnetic lenses may be readilyreproduced. Additionally, using a magnetic field sensor may eliminatethe need to couple a current sensing resistor to the control circuit.Therefore, the magnetic lens may be driven with the same current but byapplying a lower voltage to the magnetic lens.

EXAMPLE Effect of Magnetic Field Feedback Control on a Magnetic Lens

[0102]FIG. 7 illustrates a plot of the hysteresis of a magnetic lensusing a current feedback control apparatus. The magnetic field strengthwas estimated by measuring the current in the lens coil. The current inthe lens coil was varied to alter the magnetic field strength of themagnetic lens. Variations in the current in the lens coil were highestat the beginning of testing and were decreased over time. The firstcurrent, which was applied to the lens coil, has been designated as thepoint on the graph which has been labeled “start.” The last current,which was applied to the lens coil, has been designated as the point onthe graph which has been labeled “end.” The measured magnetic field hasbeen plotted versus the desired magnetic field. The range of themeasured magnetic field and the desired magnetic field range fromapproximately −10 gauss to approximately +10 gauss. As shown in FIG. 7,the measured magnetic field has a non-linear relationship to the desiredmagnetic field. The non-linear relationship may be caused by the factthat instantaneous magnetic field strength is determined by the presentcurrent and the history of all other currents in the coil and iscommonly referred to as hysteresis.

[0103]FIG. 8 illustrates a plot of the hysteresis of a magnetic lensusing a magnetic field feedback control apparatus. The magnetic fieldstrength was estimated by measuring the magnetic field of the magneticlens using an Allegro Hall Sensor (Allegro Microsystems, Inc.,Worcester, Mass.). The current in the lens coil was varied to alter themagnetic field strength of the magnetic lens. Variations in the currentin the lens coil were highest at the beginning of testing and weredecreased over time. The measured magnetic field has been plotted versusthe desired magnetic field. The range of the measured magnetic field andthe desired magnetic field is from approximately −10 gauss toapproximately +10 gauss. As shown in FIG. 8, by using a magnetic fieldfeedback control apparatus, a linear relationship between the measuredmagnetic field and the desired magnetic field was established. Thelinear relationship between the measured magnetic field and the desiredmagnetic field indicates that the effects of hysteresis in the magneticlens were effectively minimized. Therefore, by implementing a controlmethod, which includes using a magnetic field feedback control apparatusas described herein, hysteresis in the magnetic field strength of themagnetic lens may be substantially eliminated.

[0104] It will be appreciated to those skilled in the art having thebenefit of this disclosure that this invention is believed to provide amagnetic lens having at least one pole piece which has at least twosectors and an apparatus configured to control the magnetic fieldstrength of a magnetic lens. Further modifications and alternativeembodiments of various aspects of the invention will be apparent tothose skilled in the art in view of this description. For example, thestructure of a magnetic lens may also be applied to electrostaticdevices, such as deflectors, or devices, which generate both magneticand electrostatic potentials, such as Wien filters. In addition, theapparatus, which may monitor and control a magnetic lens using magneticfield feedback control, may be integrated into any device whichgenerates a magnetic field during operation. It is intended that thefollowing claims be interpreted to embrace all such modifications andchanges and, accordingly, the specification and drawings are to beregarded in an illustrative rather than a restrictive sense.

What is claimed is:
 1. An apparatus configured to control a magneticfield strength of a magnetic lens during use, comprising: a magneticsensor disposed within a magnetic field generated by the magnetic lens,wherein the magnetic sensor is configured to generate an output signalduring use, and wherein the output signal is responsive to a firstmagnetic field strength generated by the magnetic lens; and a controlcircuit coupled to the magnetic sensor and the magnetic lens, whereinthe control circuit is configured: to receive the output signal from themagnetic sensor during use; to receive an input signal responsive to apredetermined magnetic field strength during use; to generate a controlsignal responsive to the output signal and the input signal during use;and to apply a current to the magnetic lens, wherein the current isresponsive to the control signal.
 2. The apparatus of claim 1, whereinthe magnetic lens is configured to apply a magnetic field to a chargedparticle beam during use.
 3. The apparatus of claim 1, wherein themagnetic lens is coupled to a scanning electron microscope.
 4. Theapparatus of claim 1, wherein the input signal comprises a voltagehaving a linear relationship to the predetermined magnetic fieldstrength of the magnetic lens.
 5. The apparatus of claim 1, wherein theoutput signal comprises a voltage having a linear relationship to thefirst magnetic field strength of the magnetic lens.
 6. The apparatus ofclaim 1, wherein the control signal is responsive to a function of theoutput signal and the input signal.
 7. The apparatus of claim 1, whereinthe control circuit is further configured to apply a current to at leastone coil of the magnetic lens.
 8. The apparatus of claim 1, wherein theapplied current is effective to generate a second magnetic fieldstrength within the magnetic lens, and wherein the second magnetic fieldstrength is closer to the predetermined magnetic field strength than thefirst magnetic field strength.
 9. The apparatus of claim 1, wherein theapplied current is effective to generate a second magnetic fieldstrength within the magnetic lens, and wherein the second magnetic fieldstrength is substantially the same as the predetermined magnetic fieldstrength.
 10. The apparatus of claim 1, wherein the apparatus is furtherconfigured to continuously control the magnetic field strength of themagnetic lens during use.
 11. The apparatus of claim 1, wherein theapparatus is further configured to intermittently control the magneticfield strength of the magnetic lens during use.
 12. The apparatus ofclaim 1, wherein the magnetic sensor is disposed within a magneticfringe field area of the magnetic lens.
 13. The apparatus of claim 1,wherein the magnetic sensor is disposed within a cavity in the magneticlens, and wherein the cavity is disposed between an outer pole piece ofthe magnetic lens and an inner pole piece of the magnetic lens.
 14. Theapparatus of claim 1, wherein the magnetic sensor is disposed within aninner pole piece of the magnetic lens.
 15. The apparatus of claim 1,further comprising a temperature sensor coupled to the magnetic lens,wherein the temperature sensor is configured to generate a temperaturesignal during use, and wherein the temperature signal is responsive to atemperature of the magnetic lens.
 16. The apparatus of claim 15, whereinthe temperature sensor is further coupled to the magnetic sensor,wherein the magnetic sensor is further configured to receive thetemperature signal during use and to generate an output signal duringuse, and wherein the output signal is further responsive to thetemperature of the magnetic lens.
 17. The apparatus of claim 1, whereinthe control circuit comprises a low-pass circuit element configured toreceive the output signal during use and to reduce fluctuations in theoutput signal during use.
 18. The apparatus of claim 1, wherein thecontrol circuit comprises an operational amplifier configured togenerate a comparison signal during use, wherein the comparison signalis responsive to a comparison of the output signal and the input signal,and wherein the control signal is further responsive to a function ofthe comparison signal.
 19. The apparatus of claim 1, wherein the controlcircuit comprises an electronic current drive system configured toreceive the control signal during use and to apply the current to themagnetic lens during use.
 20. A method for controlling a magnetic fieldstrength of a magnetic lens, comprising: generating an output signal inresponse to a first magnetic field strength generated by the magneticlens; generating an input signal in response to a predetermined magneticfield strength; generating a control signal in response to the outputsignal and the input signal; and applying a current to the magneticlens, wherein the current is responsive to the control signal.
 21. Themethod of claim 20, further comprising directing a charged particle beamthrough the magnetic lens such that the magnetic field is applied to thecharged particle beam.
 22. The method of claim 20, wherein the magneticlens is coupled to a scanning electron microscope.
 23. The method ofclaim 20, wherein the input signal comprises a voltage having a linearrelationship to the predetermined magnetic field strength.
 24. Themethod of claim 20, wherein the output signal comprises a voltage havinga linear relationship to the magnetic field strength of the magneticlens.
 25. The method of claim 20, wherein the control signal isgenerated in response to a function of the output signal and the inputsignal.
 26. The method of claim 20, wherein applying a current to themagnetic lens comprises applying a current to at least one coil of themagnetic lens.
 27. The method of claim 20, wherein applying a current tothe magnetic lens comprises generating a second magnetic field strengthwithin the magnetic lens, and wherein the second magnetic field strengthis closer to the predetermined magnetic field strength than the firstmagnetic field strength.
 28. The method of claim 20, wherein applying acurrent to the magnetic lens comprises generating a second magneticfield strength within the magnetic lens, and wherein the second magneticfield strength is substantially the same as the predetermined magneticfield strength.
 29. The method of claim 20, further comprisingperforming the method continuously.
 30. The method of claim 20, furthercomprising performing the method intermittently.
 31. The method of claim20, further comprising generating a temperature signal in response to atemperature of the magnetic lens.
 32. The method of claim 20, furthercomprising generating a temperature signal in response to a temperatureof the magnetic lens and generating the output signal in response to thetemperature of the magnetic lens.
 33. The method of claim 20, furthercomprising reducing fluctuations in the output signal using a low-passcircuit element.
 34. The method of claim 20, further comprisinggenerating a comparison signal in response to a comparison of the outputsignal and the input signal, wherein generating the control signalcomprises applying a gain to the comparison signal.
 35. A systemconfigured to inspect a specimen during use, comprising: at least onemagnetic lens configured to apply a magnetic field to a charged particlebeam during use, wherein the magnetic lens is positioned along a path ofthe charged particle beam; and an apparatus configured to control amagnetic field strength generated by the magnetic lens during use,wherein the apparatus is coupled to the magnetic lens and the system,the apparatus comprising: a magnetic sensor disposed within the magneticfield generated by the magnetic lens, wherein the magnetic sensor isconfigured to generate an output signal during use, and wherein theoutput signal is responsive to a first magnetic field strength generatedby the magnetic lens; and a control circuit coupled to the magneticsensor and the magnetic lens, wherein the control circuit is configured:to receive the output signal from the magnetic sensor during use; toreceive an input signal responsive to a predetermined magnetic fieldstrength during use; to generate a control signal responsive to theoutput signal and the input signal during use; and to apply a current tothe magnetic lens, wherein the current is responsive to the controlsignal.
 36. The system of claim 35, wherein the system comprises ascanning electron microscope.
 37. The system of claim 35, wherein thespecimen is fabricated using a semiconductor manufacturing process. 38.The system of claim 35, further comprising a charged particle beamsource configured to produce the charged particle beam during use. 39.The system of claim 35, further comprising a stage configured to supportat least a portion of the specimen during use.
 40. A method forinspecting a specimen, comprising: generating a magnetic field by amagnetic lens and applying the magnetic field to a charged particlebeam, wherein applying the magnetic field to the charged particle beamcomprises directing the charged particle beam through the magnetic lens;and controlling a magnetic field strength of the magnetic lens,comprising: generating an output signal in response to a first magneticfield strength generated by the magnetic lens; generating an inputsignal in response to a predetermined magnetic field strength;generating a control signal in response to the output signal and theinput signal; and applying a current to the magnetic lens, wherein thecurrent is responsive to the control signal.
 41. The method of claim 40,wherein the magnetic lens is coupled to a scanning electron microscope.42. The method of claim 40, further comprising fabricating the specimenusing a semiconductor manufacturing process.
 43. The method of claim 40,further comprising generating the charged particle beam using a chargedparticle beam source.
 44. The method of claim 40, further comprisingpositioning at least a portion of the specimen on a stage prior to saiddirecting.